Question
My carnation seedlings have been outside for at least a month and it had no problems. But just a few days ago the whether got warm, one of the biggest one started wilting when under the sun. The other ones are fine. The top leaves become really soft and thin. When I take it out of the sun, it usually restores to its original shape but the leaves are still thin and soft. I don't understand I really like this particular seedling, why is it wilting and how can resolve this problem?

Answer
Kate,
You don't say where you are and how warm the weather is, but carnations (all Dianthus) grow best in cool weather and alkaline soil. If your seedlings are in a pot with potting soil your soil is probably OK, but these plants don't do well in heat or humidity.  Are you planning to put them in the ground instead of keeping them in pots? If so, plant them where they are in morning sun but shade after 1 PM if you live where it gets really hot.  These plants also appreciate good drainage. If you are growing the true florist carnations, (Dianthus caryophyllus) know that they are hard in gardens and are best grown in greenhouses with controlled climates.  

Also, be sure that you aren't overwatering. If a plant is in too much water the roots rot and so there isn't enough of a root system to absorb the water when they need it. When people see the plant wilting (because of this root rot) they think the plant needs water and they give it more. In this case the foliage becomes weak and limp because the roots can't absorb the water needed to keep them plump. If you think that this might be the problem, you could either stop watering and see if the plant recovers (sometimes it's too late) or take the plants out of the soil and gently knock off some of the old soil and repot the plant in fresh soil. Water once after repotting and then let the soil dry out before watering again.

I hope this helps,
C.L.